@stuartk so you’ve pretty much narrowed it down to DAC. I wouldn’t worry about silver in digital cable. It won’t impact the sound the same way the silver in analog cables would.
My conclusion is your dac/amp/speakers combination is not very synergistic.
The DAC is emphasizing the sibilance and the amp is exacerbating it.
At this point no amount of vibration control, tweaks or cables will fix this issue. At best it would provide a temporary bandaid. Probably time to try a new DAC or a new amp. Room dampening might help as well if you can add some.